I confirmed the behavior with packer built VMs running Ubuntu (12.04, 13.10, and 14.04 (daily build)). Building the same machines in VirtualBox 4.3.10 with guest additions 4.3.8 didn't have the error and performed as expected.

A VBox.log file from the Ubuntu 13.10 machine with the error is attached.

I don't know what happened to this new version of Guest Additions, but it seems that it missed a symlink during installation. This breaks the path of /sbin/mount.vboxsf so that the mount command fails.

Can someone give an ETA for the next release? We are also using vagrant/ansible a lot and this bug is really annoying to hotfix it over and over again.

The list of builds is not up to date at the 4.3.x page but generally speaking there was a release every month or so. 4.3.10 was released on 2014-03-25 (so just over a month ago). Is it expected to have 4.3.11 released within a few days now? The patch has been applied in svn four weeks ago and I guess a lot of people have stumbled upon this issue :)